If the food source was plentiful enough I'd think any cetacian or pinniped could live in freshwater. The food would have to be there. There are no marine mammals of any size +/- 50kilos in the great lakes because there is no food source. Otter and mink and other intercoastal species don't really count in this because I don't consider them marine mammals, even though they rely on it or their own survival.